1. Field of the Invention
The present invention relates to an x-ray tube for generating x-rays, an x-ray generator, and an inspection system for an object to be inspected using them.
2. Related Background Art
Known as a conventional x-ray tube is one incorporating therein an electron gun for emitting electrons and a target for generating x-rays in response to the electrons as described in Japanese Patent Application Laid-Open No. HEI 7-296751. On the other hand, known as a conventional x-ray generator is one incorporating therein an x-ray tube, a driving circuit for the x-ray tube, and the like as described in Japanese Patent Application Laid-Open No. HEI 7-29532.
Such x-ray tube and x-ray generator are mainly used for nondestructive/noncontact observation of internal structures of objects and the like as described in Japanese Patent Application Laid-Open No. HEI 6-315152. For example, an object to be inspected is irradiated with x-rays emitted from the x-ray tube and x-ray generator, and the x-rays transmitted through the object are detected by an x-ray/fluorescence multiplier (an image intensifier tube: I.I. tube) or the like. Then, the resulting magnified penetration image of the object is observed, whereby the nondestructive/noncontact observation of internal structure of object becomes possible.
In general, as described in Japanese Patent Application Laid-Open Nos. HEI 6-94650 and HEI 6-18450, such an inspection of the object to be inspected employs a technique in which the object is rotated about an axis orthogonal to the direction in which the x-rays are emitted, so as to change the orientation of the object, thereby accurately specifying a defective site.
On the other hand, the magnification rate of the penetration image is determined by the ratio between the distance (A) from the x-ray generating position (the focal position of the x-ray tube) within the x-ray tube apparatus to the position of the object and the distance (B) from the position of the object to the x-ray entrance surface of the I.I. tube. That is, the magnification rate M is expressed byM=(A+B)/A.  (1)Normally, A<<B, and therefore the expression (1) can be represented byM=B/A.  (2)
Namely, for yielding a greater magnification rate, decreasing A or increasing B may be considered. Increasing B, however, not only enhances the overall size of the x-ray inspection apparatus, but also remarkably increases its weight by requiring a greater amount of lead shield for keeping the x-rays from leaking outside, and so forth.
Therefore, it is desirable that A be as small as possible. In the case using a technique in which the orientation of the object to be inspected is changed as mentioned above, however, a sample holder for mounting the object or the like may come into contact with the exit surface of the x-ray tube if A is made smaller. Consequently, there is a certain limit to increasing the magnification rate of penetration image. Hence, it has been difficult to accurately inspect the state of an object to be inspected while observing a penetration image thereof with a high magnification rate.